1. Technical Field
The present invention relates to a radio communication apparatus and a radio communication method.
2. Background Art
3GPP-LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. According to 3GPP-LTE, a radio communication base station apparatus (hereinafter abbreviated as a “base station”) transmits a reference signal (“RS”) using predetermined communication resources and a radio communication terminal apparatus (hereinafter abbreviated as a “terminal”) performs channel estimation using the received reference signal and demodulates data (see Non-Patent Document 1).
Furthermore, when the base station is provided with a plurality of antenna ports, the base station can carry out diversity transmission. On the other hand, in order for a terminal to receive a diversity-transmitted signal without errors, the terminal needs to know the conditions of the propagation path from the antenna port group used in the base station to the terminal. Therefore, RSs need to be transmitted from all antenna ports provided in the base station without mutual interference. To realize this, 3GPP-LTE adopts a method of transmitting RSs using different timings and carrier frequencies in the time domain and the frequency domain from individual antenna ports in the base station.
FIG. 1A illustrates a configuration of a base station having two antenna ports (2-Tx base station) assumed in 3GPP-LTE and FIG. 1B illustrates an RS transmission method by a 2-Tx base station. Likewise, FIG. 2A illustrates a configuration of a base station having four antenna ports (a 4-Tx base station) assumed in 3GPP-LTE and FIG. 2B illustrates an RS transmission method by a 4-Tx base station. In FIGS. 1B and 2B, the vertical axis (frequency domain) shows subcarrier units and the horizontal axis (time domain) shows Orthogonal Frequency Division Multiplexing (OFDM) symbol units. Furthermore, one slot is made up of seven OFDM symbols. Furthermore, R0, R1, R2 and R3 indicate. The RSs transmitted from antenna ports 0, 1, 2 and 3 (the first, second, third and fourth antenna ports). Furthermore, a unit of one block enclosed by a frame of a bold line (12 subcarriers in the frequency domain, seven OFDM symbols in the time domain) will be referred to as a “resource block (“RB”).” As is clear from FIGS. 1B and 2B, the 4-Tx base station reduces the frequency of RS transmission from antenna port 2 (third antenna port) and antenna port 3 (fourth antenna port) to minimize the overhead on RS transmission.
By the way, a 1-Tx base station transmits RS using the only resources of R0 in the RS arrangement by the 2-Tx base station.
As described above, the 4-Tx base station has a low transmission frequency of RSs from antenna port 2 and antenna port 3. Therefore, a terminal that receives RSs from the 4-Tx base station cannot interpolate the channel estimate values of antenna port 2 and antenna port 3 in one RB, and, consequently, the accuracy of channel estimation is deteriorated severely during high-speed movement. Therefore, it has been confirmed that avoiding the use of antenna port 2 and antenna port 3 of the base station during high-speed movement of the terminal can improve the SNR performance at the terminal (see Non-Patent Document 2).
Therefore, although a 4-Tx base station has been conventionally provided with four antenna ports, only two antenna ports are used when a terminal is moving at high speed.
Alternatively, in order to use four radio transmitting sections of a 4-Tx base station effectively, as shown in FIG. 3, conventionally, a 4-Tx base station is conventionally handled as a virtual 2-Tx base station provided with virtual antenna 0 made up of antenna port 0 and antenna port 2 and virtual antenna 1 made up of antenna port 1 and antenna port 3. However, in FIG. 3, a CDD (Cyclic Delay Diversity) generation section is added to antenna port 2 and antenna port 3 to suppress unnecessary beam forming effect caused by virtual antennas.
In this case, assuming that a signal outputted from the mapping section in FIG. 3 is:
                              S                      2            ⁢                                                  ⁢            Tx                          =                  (                                                                      S                  0                                                                                                      S                  1                                                              )                                    [        1        ]            the signal yvirtual transmitted from the four antenna ports is represented by:
                              y          virtual                =                              D            ⁡                          (                                                                    10                                                                                        01                                                                                        10                                                                                        01                                                              )                                ⁢                      s                          2              ⁢                                                          ⁢              Tx                                                          [        2        ]            where D is a 4×4 diagonal matrix representing CDD.    Non-Patent Document 1: 3GPP TS 36.213 V1.1.0, R1-072633.    Non-Patent Document 2: Transmit Diversity Scheme for Control Channel in E-UTRA, R1-072423.